1. Field of the Invention
The present disclosure relates to process liquid coating apparatuses and image forming systems for coating a recording medium with a process liquid.
2. Description of the Related Art
Among methods for recoding images by inkjet, there is a method in which process liquid for aggregating ink is applied to a sheet serving as a recording medium just before the ink droplet adheres to the sheet, thereby improving image quality.
A method for applying the process liquid is known, in which entire sheet surface is coated with the process liquid by using a roller. In FIG. 19, an example configuration of a coating unit, which is an example of a coating apparatus for coating the sheet with the process liquid by using the roller, is shown. In FIG. 19, “W” indicates a recording medium such as a paper sheet, 90 indicates a process liquid coating unit, 91 indicates a chamber for the coating liquid, “L” indicates process liquid, 92 indicates a squeeze roller, 93 indicates coating roller, and 94 indicates pressure roller. In the example, the process liquid L is pumped up through rotation of the squeeze roller 92 driven by a motor. The process liquid L pumped up by the squeeze roller 92 is scrapped off by a nip of the coating roller 93 and the squeeze roller 92 whose edge portions are covered with elastic body such as gum elastic, whereas the remained coating liquid is thinly and uniformly drawn out on a surface of the coating roller 93. The process liquid L drawn out on the coating roller 93 is transferred to a sheet caught in a coating nip N configured by the pressure roller 94 and the coating roller 93 (for example, Patent Document 1).
In the above described liquid coating apparatus for inkjet printer, in a case where the liquid is applied to continuous paper such as continuous roll of paper, different paper sagging may occur in sheet width direction due to variance of sheet (recording medium) property or thickness in sheet width direction. In FIG. 20, waviness W1 indicates a shape of the recording medium W at an upstream side of the coating nip N, where wave shape is formed due to the variance of thickness in sheet width direction, or the like. In a case where the waviness W1 is formed in the recording medium W, the waviness W1 of the recording medium W is pressed in the coating nip N nipping the recording medium W by the coating roller 93 and the pressure roller 94 to form wrinkle W2. The wrinkle W2 downstream of the coating nip N indicates a shape of the recording medium W after being pressed by the coating nip N.
Generally, it is known that wrinkling can be suppressed by causing a large tension of the sheet in the coating operation to suppress the paper sagging. However, tension of the sheet cannot be enlarged so much due to a condition of the device, and the like. Also, the large tension cannot be applied to some types of the sheet (e.g. thin or light sheet) due to a lack of paper strength. Therefore, occurrence of wrinkles due to variance of paper properties cannot be prevented by controlling only the tension.
In FIG. 21, another example configuration of the coating unit 90A is shown as an example of conventional technology (Patent Document 2). In the aforementioned configuration, an application amount of liquid to the sheet W is adjusted by winding the sheet around the coating roller 93 by using the winding unit 95. However, also in the aforementioned configuration, the occurrence of wrinkles cannot be perfectly prevented because the waviness W1 of the recording medium W is pressed in the coating nip N to form the wrinkle W2 in a case where the waviness W1 is formed upstream of the coating nip N as shown in FIG. 20.